This invention relates to an electrically insulating substrate having high thermal conductivity and high electric resistivity and also to a method of making such a substrate.
The semiconductor industry has made a remarkable progress in recent years and a number of circuit constituents such as semiconductor chips have been formed in increasingly higher density on an insulating substrate used in large scale integration circuit and the like. Demands for devices having greater capacity with smaller size have also become keener and insulating substrates having high thermal diffusivity have been required.
As materials for such an insulating substrate, there has conventionally been employed an alumina sintered body. Since the alumina sintered body does not have satisfactory thermal diffusivity, however, development of an insulating substrate having higher thermal diffusivity has been requested. The insulating substrate for such an application must satisfy the following requirements.
(1) high electrically insulating property, PA0 (2) high thermal conductivity, PA0 (3) its coefficient of thermal expansion is approximate to that of silicon, and PA0 (4) high mechanical strength.
A sintered body of silicon carbide has a coefficient of thermal expansion of 3.7.times.10.sup.-6 /.degree.C. which is smaller than the coefficient of thermal expansion of alumina, i.e., about 8.times.10.sup.-6 /.degree.C. and is approximate to that of silicon, i.e., about 3.3.times.10.sup.-6 /.degree.C. As to the mechanical strength of silicon carbide, its bending strength is at least 50 Kg/mm.sup.2, the value being by far higher than that of alumina, i.e., ca. 25 to 30 Kg/mm.sup.2. Further, the thermal conductivity of silicon carbide sintered body is 0.1 to 0.2 cal/cm.sec..degree.C. which is at least three times that of alumina.
Silicon carbide is a semiconductor compound of the Group IV--IV consisting of carbon and silicon. For this reason, it has been believed difficult to obtain a high density sintered body of silicon carbide with high electrical resistivity and as a matter of fact, no such sintered body has been found to this date.
On the other hand, it has been known well that since silicon carbide is a compound having high covalent bond, it is hard and tough and is stable both in oxidation resistance and corrosion resistance even at a temperature of 1,500.degree. C. or above. Due to this strong covalent bond, however, it is a material from which a high ensity sintered body can not be obtained easily.
Various sintering aids have been employed in order to obtain a high density sintered body of silicon carbide. U.S. Pat. No. 4,172,109, for example, discloses a sintered body of silicon carbide which is sintered while Be is added as the aid. This prior art relates to a high strength material obtained by sintering the raw powder of silicon carbide which contains 0.5 to 5 wt.% of excessive carbon. However, the sintered body thus formed has small electric resistivity and can not be used as an electrically insulating material.